1. Field of the Invention
This invention relates to digestion with chemical systems which form passivating layers, and more particularly to the use of ultrasonic comminution to enhance the reaction rates by the removal of passivating layers.
2. Description of the Related Art
Hydrofluoric acid is routinely used for the dissolution and digestion of various silicate and other mineral samples and of glasses for chemical analysis, for processing and for other uses where is it is necessary to solubilize the material. Passivating layers may sometimes form in the digestion of certain minerals (such as the tungsten containing mineral, scheelite), often containing high levels of calcium. In systems where passivating layers form, the digestion rate can slow dramatically and it is necessary to take steps to destroy or de-passivate this layer. Typically the steps are the addition of additional acid or of other ions that complex free fluoride so as to make this layer soluble. This is often undesirable as this means adding materials to solution which have to be removed, recycled or recovered.
U.S. Pat. No. 4,168,295, issued to H. T. Sawyer, discloses an apparatus for enhancing chemical reactions in a constantly flowing stream of liquid character fortified with oxidizing agents, such as air or oxygen or other chemical agents. The apparatus makes use of an inner resonant tube concentrically mounted within an outer resonant tube with the walls of the tube spaced from each other forming an annular passageway for the flow of liquid from one end of the passage to the other.
U.S. Pat. No. 4,879,011, issued to C. J. Schram, discloses supporting particulate matter in a fluid medium by means of an ultrasonic standing wave while a reaction is affected or controlled involving the material so supported, for example with the fluid medium or other material contained in the medium. In a preferred arrangement, the standing wave is established by opposed ultrasonic transducers producing convergent beams that compensate for attenuation of the ultrasonic energy in the fluid medium, and operating in the near field.
U.S. Pat. No. 3,525,606, issued to an A. G. Bodine, discloses a method and apparatus for leaching and extracting materials comprised of utilizing a tank to which is fed the composition to be leached in the leaching solution. Acoustically coupled to the tank is an orbiting-mass oscillator with a suitable resonator element so as to effect high resonant sonic energy within the tank, to the degree that cavitation of the liquid is obtained.